Human Granulocytic Ehrlichiosis (HGE) is a recently emergent tick-transmitted infectious disease that has been reported with increasing frequency during the last decade. The causative agent is classified within the ehrlichial genogroup II and is considered to be a variant or subspecies of Ehrlichia phagocytophila. Although many and perhaps most of the human infections are either subclinical or resolve without diagnosis, severe disease does result in prolonged hospitalization with intensive care and can progress to cause death. The manifestations of human disease are not specific but may manifest as a toxic shock-like illness, adult respiratory distress syndrome (ARDS), and fatal cases have an association with opportunistic infections. A similar association has been reported with the domestic animals infected by E. phagocytophila group organisms. The natural target cell is primarily neutrophils, although premyelocytic cells can be infected in vitro and myeloid progenitors have been hypothesized to be a site of infection in vivo. In contrast, infection of macrophages and monocyte-like cell lines is abortive and the organisms are killed. Interestingly, the severity of disease in humans does not reflect a high circulating ehrlichial load. Previous data from animals infected with E. phagocytophila indicates impaired neutrophil function - including chemotaxis, phagocytosis, and killing. These "deactivation" phenotypes may be concomitantly associated with "activation" in which binding to endothelial cells occurs and culminates in release of proinflammatory cytokines by endothelial cells and especially macrophages, leading to the triggering of a toxic-shock like injury. The PI proposes the following hypotheses: 1) E. phagocytophila group ehrlichiae bind to CD15-associated granulocyte surface receptors and initiate endocytosis via their major surface protein (MSP) antigens; 2) Binding, internalization, and propagation of E. phagocytophila group ehrlichiae initiate cytokine/chemokine expression and changes in granulocyte activation/inactivation and function; and 3): E. phagocytophila group ehrlichiae potentially influence host cell function by expressing proteins that directly interact with DNA regulatory components in the host cell's chromosomes. These hypotheses will be tested using five specific aims: i) demonstrate the morphology of the ehrlichia-host cell membrane interaction in HL60 cells in various differentiated states; ii) characterize the role of E. phagocytophila group MSPs as adhesins and the role of CD15-associated structures as the major surface receptors by which ehrlichiae attach and enter cells; iii) characterize the effects of ehrlichial binding and internalization on host granulocyte cell surface adhesion molecule expression, granulocyte-endothelial cell adherence, phagocytic activity, activation/deactivation, microbial killing, and cytokine/chemokine expression; and iv) identify the chromosomal ligand for the E. phagocytophila group ankyrin protein, EPANK1.